1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, or an MFP having functions of at least two of them.
2. Description of the Related Art
Conventionally, the image forming apparatus in the above form that utilizes the electrophotography has been widely known. Its image forming process includes forming an electrostatic latent image on the surface of the photosensitive drum that is an image carrier, developing the electrostatic latent image on the photosensitive drum to visualize it as an image by the toner that is a developer, and transferring the developed image to a recording medium by the transferring device so as to cause the toner image to be carried. Then, the toner image that has not been fixed on the recording medium is pressed/heated by a fixing device to fix the toner image on the recording medium.
The fixing device comprises a fixing member and a pressing member, and the unfixed image is heated while being held by these fixing member and pressing member, which causes the developer, in particular, the toner included in the unfixed image to be melted and softened and penetrated into the recording medium. Thereby, the toner is fixed to the recording medium.
In this type of fixing devices, when the fixing member is heated up to a predetermined temperature by a heat source, with sufficiently short heating time before the predetermined temperature is obtained, the preheating process under a standby state can be omitted. Consequently, the consumption energy can be significantly reduced. In order to achieve this, as the fixing member, members with low heat capacity, such as a thin roller or a belt comprising a metal base member and an elastic rubber layer, have been widely used. Further, for the heat source, rapid heating has been realized by the use of the IH system having higher heating efficiency, such as a ceramic heater, as well as a halogen heater that heats the fixing member by radiant heat. The fixing devices having these arrangements are disclosed in Japanese Patent Application Laid-open No. 2007-79040, Japanese Patent Application Laid-open No. 2010-32625, Japanese Patent Application Laid-open No. 2007-334205, and Japanese Patent Application Laid-open No. 2008-129517, for example.
A heated area heated by a heating source and a fixing nip are different in position, among these fixing devices, in particular, a device in which a fixing belt is configured to be hanged between a fixing roller and a heating roller, a device in which heating is made by the IH system, and a device that locally heats the fixing member by offsetting the setting position of an embedded halogen heater (partial heating system). Therefore, even if the fixing member has the heated area that is heated to a relatively high temperature, the heat of the fixing member is transferred to the recording medium passing through the fixing nip portion in performing the image fixing operation, and thus the temperature of the fixing member may not be extremely high. However, under a state where the rotation of the fixing member stops such as at the time of completion of the image fixing operation, the remaining heat (residual heat) of the heating source may cause the fixing member to be in an overheating state even if the power supply to the heating source is stopped (when sheets are in a successive conveyance, much larger heat is accumulated inside the fixing device). Alternatively, even when the remaining heat of the heating source does not cause a big problem, the remaining heat of a reflector, a stay, or inner air heated to a high temperature may cause the temperature of the surface of the fixing member to rise after the rotation stops. Further, in a case where the heated area and the fixing nip portion are at a distance, partial heating with a relatively high temperature is made at the heated area so that the necessary heat can be obtained at the time when a part of the belt heated by the heated area moves to the fixing nip portion. Therefore, unless the heat of the part of the belt is dispersed, that part of the belt will be damaged. The fixing member is likely to have such problems, in particular, in the fixing device comprising a fixing member that is further thinned to have lower heat capacity for the reduction of warm-up time or the reduction of consumption energy.
When the image forming apparatus in which the fixing device of the partial heating system as described above is installed suddenly stops due to an occurrence of abnormality, only the portion which faces to the heating source of the fixing member will be heated, causing an uneven temperature distribution in the circumferential direction of the fixing member. Therefore, in the fixing member, a difference in thermal expansion occurs between the portion facing to the heating source and the portion not facing to the heating source. As a result, the portion facing to the heating source is forced to expand in the axial direction, while the portion not facing to the heating source is maintained. Also in the portion facing to the heating source, the portions near both ends in the axial direction of the fixing member lose their heat and thus have a lower temperature than the center portion in the axial direction. Thus, the center portion in the axial direction of the portion facing to the heating source of the fixing member has the thermal expansion to the highest degree. Therefore, the center portion of the portion facing to the heating source of the fixing member is highly forced to expand outward in the axial direction while the portion not facing to the heating source is maintained. Thus, the center portion of the portion facing to the heating source is unable to expand outward, which causes so called kink that is a plastic deformation to warp inward. There has been a problem that the kink occurring in the fixing member then develops to an abnormal image and further causes the fixing member to be broken.
Therefore, there is a need for an image forming apparatus that does not cause the breakage and the like of the fixing member even if an overheating occurs in the fixing member when the fixing device suddenly stops due to the occurrence of the abnormality.